1. Field of the Invention
The present invention relates to gas separation membranes, and, more particularly, to a composite gas separation membrane including synthetic, cellulose acetate membrane supported on a synthetic polymeric fabric.
2. Description of the Prior Art
It has been recognized for many years that non-porous polymer films exhibit a higher permeability toward some gases than towards others. As early as 1831, investigations were reported on the phenomenon of enrichment of air with rubber membranes; however, not until 1950 had the practical possibility of this and other gas separations with permselective membranes been seriously studied. Weller and Steiner in their classic papers, (J. Appl. Phys., 21, 79 (1950); Chem Eng. Prog., 46, 585 (1950), demonstrated the feasibility of separating oxygen from air and described practical processes for separation of hydrogen and helium from methane. Although their results were highly valuable in the development of the science of membrane separation, the calculated membrane area requirements for industrial processes were enormous.
The technical breakthrough in the application of membranes to gas separation came with the development of a process for preparing cellulose acetate membranes in a state which retains the permselective characteristics of ordinary cellulose acetate but which yields vastly increased gas permeability. These cellulose acetate membranes are prepared from a solution of the polymer which is cast on a smooth surface then set or gelled in an ice-water bath. At this stage the membranes are heated in water to improve their selectivity characteristics and are then dried by a solvent exchange technique. The reason for the high permeability values, together with the permselective characteristics of ordinary cellulose acetate, is the formation of an "active" layer on the air-dried surface of the membrane. This active layer has characteristics similar to those of ordinary cellulose acetate and has a thickness of the order of 0.2 micrometers (.mu.m) or less, whereas the total membrane thickness may range from approximately 75 to 125 .mu.m. Thus, the membranes are said to be asymmetric. The major portion of the membrane is an open-pore sponge-like support structure through which gases may flow freely. The permeability and selectivity characteristics of these membranes are functions of casting solution composition, film casting conditions, and post-treatment and are relatively independent of total membrane thickness.
Membranes made for gas separation from cellulose acetate, or any other brittle polymer, are difficult to handle in large pieces due to their propensity to crack when in the dry state. This difficulty may be overcome partially by casting the membrane on a fabric support. The current technology for reverse-osmosis (R.O.) membranes utilizes a Dacron polyester cloth support in order to meet the requirement of high wet strength for water desalination. However, R.O. membranes are stored wet or are manufactured with an aqueous fugitive plasticizer. Neither the wet, nor the plasticized membrane are capable of gas separation. The Dacron fabric utilized in the R.O. membrances, however, exhibits very low shrinkage during the membrane processing with the result that when dried the membrane curls extensively along the edges due to the large difference in shrinkage between the membrane and the fabric. This curling makes it very difficult to fabricate gas separation spiral elements without incurring wrinkles in the membrane and cracking along the edges.